The present invention relates to a flat polarization converter and to liquid crystal display (LCD) projection system designs including the novel flat polarization converter. More specifically, the present invention relates to a compact planar polarization converter for use, for example, in full-color large-diagonal LCD projection systems. Large diagonal LCD devices are defined as those devices having diagonal dimensions greater than 150 millimeters. The polarization converter of the present invention includes planar components to efficiently produce linear polarized light over a large area from an unpolarized light source.
Some single-panel LCD devices, and in particular projections systems, require polarized light. An efficient planar polarization device would aid greatly in the design of compact and portable LCD devices.
Unpolarized light includes a linear component and an orthogonal component. A common method for producing polarized light for an LCD projection panel, comprises of the use of a polarizing beam splitter (PBS) cube. The linearly polarized component light is transmitted by the PBS cube and directed to the LCD panel, while the orthogonal component is reflected away in a perpendicular direction. Another common method for producing polarized light comprises the use of an absorbing dye or iodine based polarizer film positioned between the light source and the LCD panel. The absorbing film transmits a single component linear polarized light in one direction, while absorbing the orthogonal component. The absorbing polarizer film is often integrally incorporated into the commercial LCD panel. Alternatively, a separate polarizer plate may be positioned between the light source and LCD.
Both the PBS cube and the absorbing polarizer methods are inefficient, in that a maximum of only one-half of the available light from the source is converted to polarized light for transmission through the LCD panel. Attempts have been made to recycle the reflected polarization component from a PBS cube. However, solid glass PBS cubes are bulky and impractical for compact or planar applications.
Recently, reflective polarizing sheet films have been developed. Use of a reflective polarizing sheet film, instead of an absorbing sheet polarizer, allows for the possibility of reflecting back the s-polarization component of a light beam in the direction of the light source. Methods have been described that return the reflected polarized light to a spherical reflector behind the light source, and back to the LCD panel. However, these methods require extremely precise alignment of the optical components for efficient recycling of the light. Also, these methods again are not easily suitable for compact applications.
Other systems attempt to improve efficiency by recycling the reflected polarized light from various types of polarization producing films without returning the light to the light source. Some of these systems use polarization conversion devices that use holographic optical elements to separate the polarization components. All of these systems can take up considerable space and are not suitable for compact applications or for large-gate LCD panels.
Recently, systems have been described that convert and recycle polarized light within a plate-like element.
FIG. 1 illustrates an earlier plate-like polarization converter 10 illustrated in U.S. Pat. No. 5,566,367. A beam of incident unpolarized and collimated light 20 is compressed into collimated sub-beams 22 by a lenticular element 30 including a converging microlens 32 and a diverging microlens 34. The sub-beams 22 are incident upon a second prismatic element 40. Linearly polarized beams 24 exit the prismatic element 40. The prismatic element 40 includes a first incidence side prism 42 having a series of quarter-wave retarder films 44 and reflective polarization beam splitting coatings 46. Total reflection mirrors 48 are formed on the contact surface of side prisms 50. In this polarization converter 10, precise registration between the converging microlens 32 and a diverging microlens 34 is required. More importantly, precise registration between the lenticular element 30 and the second prismatic element 40, and more precisely, between the diverging microlens 34 and the first incidence prism 42 is necessary for efficient operation. Likewise, precise thickness control of the elements is similarly required. These precise alignment and thickness requirements, combined with the deposition of the required coatings on selective prismatic surfaces, present significant manufacturing challenges.